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Effects of pre-strain on twinning behaviors in an extruded Mg-Zr alloy
Materials Science & Engineering A 766 (2019) 138335
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Effects of pre-strain on twinning behaviors in an extruded Mg-Zr alloy a
Xin Wan , Jing Zhang a b
a,b,∗
a
, Xueyan Mo , Yu Luo
T
a
College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China National Engineering Research Center for Magnesium Alloys, Chongqing, 400044, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Pre-strain Twin aspect ratio Twin density Twinning behaviors
For Mg alloys, regulation of size and quantity of twins is important to improve their performance. With respect to twin regulation, pre-strain (e.g., direct compression and compression–tension) is recognized as a reliable method. In the present study, the further investigation about the influence of different pre-strain loading conditions on the evolution of the subsequent twinning behavior was carried out via statistical viewpoint and in-situ EBSD observation. The results show that the dominant deformation mechanism can be changed by different prestrains. In the direct compression pre-strained sample, the subsequent deformation is dominated by twin growth. However, subsequent deformation is dominated by twin nucleation in the compression–tension pre-strained sample with complex pre-strain histories. Additionally, the results reveal that the compression–tension pre-strain is beneficial to promote the nucleation of twins inside grains, other than on the grain boundaries, and thereby increases the density of twins. Particularly, the pre-strain suggests that the nucleation proportion of twins inside the grain exceeds that at the grain boundary, and this characteristic significantly differs from the general view. Correspondingly, experimental results and the underlying mechanism of the aforementioned special phenomenon are reported and discussed.
1. Introduction Magnesium (Mg) and its alloys are lightweight structural materials that attract significant research attention in electronic communications, aerospace, and automotive industries [1–3]. However, their widespread application encountered bottlenecks due to their low strength and ductility [4,5]. Mg alloys exhibit a limited number of independent slip systems at room temperature. Hence, deformation twinning represents an extremely important deformation mechanism in terms of mechanical properties and deformation behaviors [6–8]. Among various twinning modes in Mg alloys, {10–12} extension twin is expected to occur most easily, and this is attributed to its significantly lower critical resolved shear stress (CRSS) [6,9,10]. The main role of twinning in plastic deformation includes reorientation of the twinned regions, relaxing stress concentrations, and Hall–Petch effect via twinning-induced change in grain size [11–13]. Recently, the introduction of fine and dense twin lamellae is demonstrated as an effective means to tailor the ductility and strength of metallic materials [14–16]. Lu et al. [14] revealed that pure electro-deposited copper with high-density nanotwins exhibits extremely high strength with significant plastic strain. Furthermore, Song et al. [15] and Xin et al. [16] reported that the fine {10–12} twin can effectively toughens and strengthens textured Mg alloys.
∗
The amount and size of {10–12} twins can be tailored by pre-strain paths and levels. For example, Ghaderi et al. [17] and Barnett et al. [18] investigated the effect of compressive strain on twin characteristic parameters and revealed that the twin aspect ratio (twin thickness over twin length) and twin number density increased with increasing strain within the considered strain range. Generally, detwinning behavior of the twinned area is easily activated during subsequent strain-pathchanged reloading or reverse loading [19,20]. Specifically, strains mediated by twinning can be absolutely recovered via detwinning during subsequent reverse loading [21]. Pre-strain paths can include direct compression and compression–tension. Differences in the complex initial microstructure are observed in different pre-strain loading conditions. Our recent study revealed that when twinning and detwinning behavior are combined, twin aspect ratio and twin incidence are regulated via controlling compression–tension pre-strain [22]. In addition to different amount and size of twins obtained via different pre-strain paths and levels, the dislocation density in the microstructure also exhibits significant differences. Although considerable efforts focused on investigating twin nucleation and evolution characteristics while considering different pre-stain paths and levels, there is paucity of investigations on how the complex initial microstructure introduced by different pre-strain loading conditions further affects subsequent
Corresponding author. College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China. E-mail address: [email protected] (J. Zhang).
https://doi.org/10.1016/j.msea.2019.138335 Received 14 June 2019; Received in revised form 24 August 2019; Accepted 26 August 2019 Available online 27 August 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
Materials Science & Engineering A 766 (2019) 138335
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solution of 10 mL acetic acid, 5 g picric acid, 70 mL ethanol for 10–15 s at ambient temperature. For each sample, five OM pictures were taken at different viewing areas of the sample. The average twin aspect ratio and twin density (N, per mm2) on the OM pictures were measured using a TCI metallographic analysis system that exhibits a very rich processing and measurement functions [22], and more than 2000 twins were extracted in each sample. For EBSD examination, a scanning electron microscope (JOEL JSM 7800F) equipped with an HKL Channel 5 System was employed. In order to reveal the deformation behavior via re-compression, in situ EBSD maps were utilized using a step size of 0.8 μm, and the magnification was set as 400. All EBSD measurements were conducted from the center of the samples. The observed surface was ground mechanically, followed by electrochemical polishing in commercial AC2 solution for 35 s at 20 V. The EBSD data were analyzed using the Channel 5 software.
twinning behaviors. It is noted that a systematic study on the effect of pre-strain loading conditions on twin evolution significantly contributes to an effective tailoring of twin microstructure. In this paper, direct compression of pre-strain and compression–tension pre-strain tests along the extrusion direction (ED) was conducted on extruded Mg-Zr rods to introduce different initial microstructures. Subsequently, re-compression tests along the ED were employed on the pre-strained samples to observe the subsequent microstructure evolution. The effects of pre-strain loading conditions on the variation in twin characteristic parameters (i.e., average twin aspect ratio and twin density) during re-compression were quantitatively investigated from a statistical viewpoint to examine the relationship between pre-strain loading conditions and subsequent twinning behavior. Furthermore, in situ electron backscatter diffraction (EBSD) tests were performed to further clarify the underlying mechanism of the twinning behavior in different pre-strained samples during re-compression.
3. Results 2. Experiments and methods 3.1. Mechanical behavior 2.1. Sample preparation and mechanical tests Stress–strain curves of the pre-strained samples under re-compression along the ED to a cumulative strain of 2% are given in Fig. 2. From these results, it can be seen that all pre-strained samples exhibit similar shapes, but the yield stresses are significantly different. The yield stresses derived from those stress–strain curves are listed in Table 2. Sample 1#-1-T experienced direct compression pre-strain; it is observed that the yield stress is at a relatively low level. Specimen 1#-2-1-T is pre-deformed via compression 2% followed by reverse tension 1%, and this led to an appreciable increment in strength (Δσ = 6 MPa). Additionally, as shown in Table 2, a higher CYSs are produced in the 1#-41-T sample (Δσ = 15 MPa) and 1#-6-1-T sample (Δσ = 20 MPa), compared to the 1#-2-1-T sample. The increment of yield stress is potentially attributed to the fact that the dislocation density in matrix which could inhibit the twin boundaries migration increases with the initial compression strain. Increment in the dislocation density increases the yield stress.
Hot-extruded Mg-0.037Zr (wt.%) alloy bar with a diameter of 16 mm was used in the present study. After annealing at 350 °C for 2 h, equiaxial twin-free grain structure with an average grain size of 29 μm was obtained in the material, as shown in Fig. 1(a). The initial texture of the material was measured using Rigaku D/Max–2500 PC X-ray diffraction machine. The results showed that the initial texture of the hot-extruded Mg bar exhibited a strong basal texture with majority of basal poles normal to the ED, as seen in Fig. 1(b). In the pre-strain tests, the specimens were firstly subjected to compression test, in which the specimens were initially compressed along ED to different plastic strain levels of 1%, 2%, 4% and 6%, and then reversely stretched or not, to a same accumulated compression strains of 1% immediately after unloading. Following this, a relatively low temperature annealing, 200 °C for 0.5 h, was conducted on the samples, which is unlikely to remove the majority of the dislocations. The purpose of this heat treatment is to reduce the internal stresses caused by complex deformation histories. In the re-compression tests, the pre-strain samples were re-compressed by 1% along the ED to a cumulative strain of 2%. The designations of the pre-strain and re-compression test samples, along with their deformation histories, are displayed in Table 1. All deformation tests were performed at a strain rate of 10−3 s−1 using an INSTRON dynamic fatigue testing machine at room temperature.
3.2. Microstructure evolution during re-compression The optical microstructures of pre-strained and re-compressed samples are shown in Fig. 3. As shown in Fig. 3(a1-d1), many lenticular residual twins (denoted in red) are observed after the pre-strain. Interestingly, the twin characteristics at the same 1% cumulative strain differ because pre-strained samples experience different deformation histories. The detailed statistics of twin characteristic parameters as calculated by TCI metallographic analysis system for different loading conditions are shown in Fig. 4. It is observed that the prior twins obtained by the compression–tension pre-strain are finer and exhibit a number density that exceeds those of the direct compression pre-strain. The optical microstructure of re-compressed samples are shown in
2.2. Microstructure and texture measurements Microstructure evolution on the cross section (RD-ND plane) was characterized by an optical microscope (Axiovert 40 MAT Zeiss) and an EBSD technique. The samples for optical microstructure (OM) examination were mechanically ground, and then chemically etched in a
Fig. 1. (a) Optical micrographs and (b) corresponding texture of the annealed extrusion alloy bar. TD-transverse direction; ND-normal direction. 2
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Table 1 Designations of the pre-strain and re-compression test samples. Sample
1#-1-T 1#-2-1-T 1#-4-1-T 1#-6-1-T 1#-1-T-2 1#-2-1-T-2 1#-4-1-T-2 1#-6-1-T-2
Pre-strain Compression strain (%)
Reverse tensile strain (%)
1 2 4 6 1 2 4 6
–a 1 3 5 –a 1 3 5
Intermediate Annealing
Re-compression (%)
Accumulated compression strain (%)
T T T T T T T T
–b –b –b –b 1 1 1 1
1 1 1 1 2 2 2 2
T indicates that the sample was subjected to intermediate annealing. a Indicates that the sample was not subjected to reverse tension deformation. b Indicates that the sample was not subjected to re-compression deformation.
the twins in 1#-1-T sample is evident during the subsequent loading as shown in Fig. 3(a1-a2). However, with respect to 1#-6-1-T sample, the growth of twins is slight although the twin density varies significantly as shown in Fig. 3(d1-d2). Additionally, as shown in Figs. 3 and 4, 1#6-1-T sample is more prone to obtain high-density fine twins than other samples after re-compression. Subsequently, in situ EBSD analysis of the microstructure is performed, and the crystallographic orientation of the samples is determined. Figs. 5 and 6 show the IPF maps before and after re-compression of 1#-1-T sample and 1#-6-1-T sample, respectively. As shown in the figures, many parallel twin lamellas appear in the pre-strained samples. The twin lamellas are identified as {10–12} extension twins. Contraction twins or double twins are barely observed. After re-compression, significant differences in the nucleation sites of twins were observed between 1#-1-T-2 sample and 1#-6-1-T-2 sample. In 1#-1-T-2 sample, most twins nucleate at the grain boundaries. Twin chains are also observed as denoted by black regions in Fig. 5(b). However, in 1#6-1-T-2 sample, there are many areas of fine twin nucleation inside the grain, as shown in black regions in Fig. 6(b).
Fig. 2. Re-compression stress–strain curves along the ED for pre-strained samples.
4. Discussion Table 2 Compression yield stress (CYS) for pre-strained samples.
4.1. Effects of pre-strain loading conditions on twinning behavior
Sample
1#-1-T
1#-2-1-T
1#-4-1-T
1#-6-1-T
CYS (MPa)
67
73
82
87
The variation in twin characteristic parameters of pre-strained samples before and after re-compression was analyzed to investigate the effect of pre-strain loading conditions on subsequent twin nucleation and growth. The results are shown in Fig. 7. As shown in the figure, increase in the average twin aspect ratio in 1#-1-T sample is approximately 0.0154, and this exceeds any other samples. This indicates that the twin growth in 1#-1-T sample is more evident than others.
Fig. 3(a2-d2). The results reveal that twin growth and new nucleated twins are observed for all samples although the extent of growth and number of new twins differ from each other. For example, the growth of
Fig. 3. (a1-d1) Optical microstructure of pre-strained samples under different loading conditions, (a2-d2) optical microstructure of the re-compressed samples. 3
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Fig. 4. Distribution of twin characteristic parameters. (a) Twin density distribution, (b) average twin aspect ratio distribution.
increase of the initial compression strain, subsequent deformation induced by the twin growth is gradually weakened, and new twin nucleation is gradually promoted. Eventually, in 1#-6-1-T sample, new twin nucleation is more likely to occur rather than twin growth, thereby indicating that subsequent deformation is dominated by new twin nucleation.
Additionally, with respect to compression–tension pre-strain samples, the increase in average twin aspect ratio gradually decreases when the initial compression strain increases. Specifically, increase in the average twin aspect ratio in 1#-6-1-T sample is extremely low and nearly half of that in 1#-1-T sample, thereby indicating an inconspicuous twin growth in 1#-6-1-T sample. In contrast to the variation in the average twin aspect ratio, the twin density increment in 1#-6-1-T sample is significant (800 N/mm2). Conversely, the twin density increment in 1#1-T sample is rare (220 N/mm2). This implies that the twin nucleation in 1#-6-1-T sample is more universal than that in 1#-1-T sample during subsequent loading. Thus, it is concluded that the variation in the twin parameters are closely related to pre-strain histories. When the pre-strain history is simple (1#-1-T), increase in the average twin aspect ratio is at a relatively high level, and increase in the twin density is at a relatively low level under re-compression. Thus, the subsequent deformation is dominant by twin growth. However, when compression–tension prestrain is employed (samples 1#-2-1-T, 1#-4-1-T, 1#-6-1-T), with the
4.2. Effects of pre-strain loading conditions on subsequent nucleation of twins 4.2.1. Nucleation site analysis of new nucleation twins As shown in in situ EBSD maps, different nucleation characteristic of twins are found in samples 1#-1-T-2 and 1#-6-1-T-2. In order to further illustrate the effect of pre-strain histories on subsequent nucleation, the nucleation sites of the new twins in the two strain paths (approximately 300 grains) are calculated based on the in situ EBSD data, and the statistical results are shown in Fig. 8. It is observed that the proportion of grain boundary nucleation in 1#-1-T sample during the re-
Fig. 5. In situ EBSD maps (a, b) and boundaries misorientation maps (c, d) of the samples: (a) and (c) 1#-1-T; (b) and (d) 1#-1-T-2. New twins are denoted by white arrows. 4
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Fig. 6. In situ EBSD maps (a, b) and boundaries misorientation maps (c, d) of the samples: (a) and (c) 1#-6-1-T; (b) and (d) 1#-6-1-T-2. The new twins are denoted by the white arrows.
different from the general view. Evidently, the more complex the path the sample experiences, the more dislocation it contains, i.e., the dislocation density in the sample increases from 1#-1-T sample to 1#-6-1-T sample. Sample 1#-6-1-T is pre-deformed via compression and tension. There are many dislocations inside grains, and they are expected to be effective sites for subsequent twin nucleation, and the nucleation sites even exceed that of the grain boundaries. Additionally, the aforementioned defects can hinder twin growth via interaction between dislocations and twin boundaries and improve yield stress [27,28]. Thus, dense and fine twins are obtained in
compression significantly exceeds grain internal nucleation, and this is consistent with the results of other studies [23–25]. The twins tend to nucleate at grain boundaries, and this is potentially due to two reasons. First, the grain boundary is prone to stress concentration during the deformation, and this creates high stress conditions to promote the nucleation of twins. Second, the grain boundary itself can generate a twin core via atomic rearrangement under certain conditions [26]. However, after 1#-6-1-T pre-strained sample is re-compressed, the proportion of grain internal nucleation for twins is nearly twice that of grain boundary nucleation. The special phenomenon is significantly
Fig. 7. Variation in twin characteristic parameters during re-compression. The orange arrows represent increases in the average twin aspect ratio, and the blue arrows represent the increase in twin density. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 5
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data are calculated, as shown in Fig. 9. In 1#-1-T sample, when a low number of twins are involved in the initial grains, the ratio of grain boundary nucleation is more than twice that of grain internal nucleation. Twin growth is easy in the sample during re-compression, and this leads to a twinning stress field capable of promoting twin nucleation at the grain boundaries via an interaction between existing twins and the grain boundaries as shown in Fig. 5. However, when the amount of prior twin increases, the proportion of grain boundary nucleation gradually decreases, and the proportion of internal nucleation gradually increases. When the number of prior twin lamellae exceeds 3, the ratio of grain internal nucleation to grain boundary nucleation does not exhibit a significant difference. The results indicate that in 1#-1-T sample when the initial grain contains a high number of twins, twin–twin interaction is prevalent and subsequent promotion of grain internal nucleation is achieved via the interaction stress field [31,32]. With respect to 1#-6-1-T sample, the sample is pre-deformed by compression corresponding to 6% and then reverse tension corresponding to 5%. When the initial grains do not contain a twin, the subsequent twin is more favorable for nucleation at the grain boundaries than matrix, and this is identical to that of 1#-1-T sample as shown in Fig. 9 (b). However, when 1 to 4 twins are involved in the grain, the proportion of the grain internal nucleation exceeds that of the grain boundary nucleation. Hence, it is expected that the prior twinning stress field significantly affects subsequent twin nucleation in the grain in 1#-6-1-T sample. When prior twins exist in the grains, subsequent twins are more likely to nucleate inside the grain under the action of the prior twinning stress field. However, grain boundaries become favorable nucleation sites for subsequent twins when the grain does not contain prior twins. In order to further illustrate the effect of prior twins and dislocation density on subsequent twinning behavior in different pre-strain paths, the evolution of two twins with similar Schmid factor (SF) in 1#-1-T sample and 1#-6-1-T sample is explored as shown in Fig. 10. The relationship between the prior twins and subsequent twin nucleation in 1#-1-T sample is shown in Fig. 10(a and b). The initial grain Ma contains a prior twin Ta0, and twin growth is evident during the subsequent loading as shown in Fig. 10(b). When Ta0 interacts with the grain boundary, the stress concentration caused by the interaction facilitates subsequent twin Tb nucleation at the grain boundary in Ma due to the high difference in the orientation of Ma and Mb grains. In 1#-6-1T sample, the relationship between the prior twins and subsequent twin nucleation is shown in Fig. 10(c and d). It is observed that grain Mc possesses a prior twin Tb0. During the re-compression, the prior twin Tb0 grows to a slight extent. Conversely, several twin nucleation
Fig. 8. Nucleation sites of new twins in different pre-strain samples under recompression.
sample 1#-6-1-T-2. However, sample 1#-1-T only experiences direct compression by 1% with a few defects in the microstructure, and this is disadvantageous to subsequent nucleation inside the grains relative to grain boundaries. Eventually, the proportion of grain boundary nucleation exceeds that of grain internal nucleation. Thus, it is concluded that differences in the dislocation density in pre-strain samples is a potentially important factor that affects subsequent twin nucleation. 4.2.2. Relationship between prior twins and subsequent nucleation of twins Stress fields close to twin tips and related local areas of an hcp polycrystal were explored by Abdolvand [29]. The results indicated that the stress field around the twin tips varied with the local neighborhood. Gu et al. [30] revealed that twins exhibited high residual stresses that significantly deviated from those in the matrix grains after compression–tension pre-strain. Furthermore, Shi et al. [31] revealed that the stress field around the twins in the pre-deformation grains is capable of affecting subsequent nucleation of twins. Therefore, it is inferred that in addition to the dislocation density, the presence of prior twins can also affect subsequent twin evolution. In order to reveal the effect of prior twins on subsequent twin formation under different prestrain paths, the prior twin numbers of the aforementioned two samples in the grains where the new twin nucleation based on the in situ EBSD
Fig. 9. Relationship between prior twin number and subsequent nucleation sites under the re-compression process (a) in 1#-1-T sample and (b) in 1#-6-1-T sample. 6
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Fig. 10. Effect of prior twins on subsequent twin nucleation under different pre-strain paths (a,b) is for 1#-1-T sample and (c, d) for 1#-6-1-T sample.
Several conclusions are reached as follows:
(Tb–Te) are observed around Tb0. Simultaneously, twin nucleation (Tf, Tg) is also observed at the grain boundary. Thus, it is concluded that the relatively complex strain path leads to many defects inside the grain, and the subsequent nucleation of twins is promoted around the existing twins along under the effect of the high twinning residual stresses. Hence, with respect to 1#-6-1-T sample, subsequent twins are more likely to nucleate inside the grains as opposed to boundaries only when prior twins are present in the grain.
(1) When 1#-1-T sample is re-compressed, the average twin aspect ratio increment in 1#-1-T sample is approximately 0.0154, and the twin density increment in 1#-1-T sample is approximately 220 N/ mm2. However, when 1#-6-1-T sample is re-compressed, the average twin aspect ratio increment in the sample is approximately half that of 1#-1-T sample, and the twin density increment in the sample is nearly four times that of 1#-1-T sample. (2) The introduction of different number of defects and twins via the change in the pre-strain path can affect the subsequent deformation mode activation. When the initial grain contains a low number of defects and twins, the subsequent deformation is dominated by twin growth. However, the subsequent deformation is dominant by twin nucleation when a high number of defects and twins are introduced into the initial grain. (3) In terms of individual twinning stress field, in 1#-1-T sample, the twinning stress field predominantly promotes subsequent twin nucleation at the grain boundaries via interaction between twins and grain boundaries. In 1#-6-1-T sample, a significant number of dislocations exist in the sample, and the high twinning stress field induced by complex pre-strain histories can improve the ability of subsequent twins to nucleate inside the grain although it does not affect the twin variant selection. Thus, in 1#-6-1-T-2 sample, the ratio of grain internal nucleation is twice that of grain boundary nucleation. (4) Following the re-loading after pre-strain, the twin density in the sample increases with increasing the initial compressive strain, while the twin size decreases with increasing the initial compressive strain. Eventually, high-density fine twins are obtained in 1#6-1-T2 sample.
4.2.3. SF analysis of new nucleation twins in 1#6-1-T-2 As described above, 1#-6-1-T-2 sample exhibits high density defects, and thus the twinning stress field generated from prior twins significantly affect subsequent twin nucleation. The SF characteristics of the new nucleation twins in the grains containing prior twins are then analyzed in 1#-6-1-T-2 sample to investigate the effect of twinning stress field and high density defects on subsequent twin variants selection, and the results are shown in Fig. 11. It is observed that the SF value of the new nucleation twins in 1#-6-1-T-2 sample is relatively high, and 85% of the SF values are distributed in the range of 0.3–0.5. Simultaneously, nearly 75% of the new nucleation twins correspond to Rank1 and Rank2. Hence, it is concluded that the newly nucleated twins also conform to the SF law. Specifically, in 1#-6-1-T sample, the prior twinning stress field and defects significantly affect the nucleation of subsequent twins while they do not significantly affect the selection of subsequent twin variants. Therefore, in a manner similar to 1#-6-1-T sample, most grains in 1#-6-1-T-2 sample also contain just a single variant as shown in Fig. 6. 5. Conclusions In the present study, a comparative study about the effect of different pre-strains on the evolution of the subsequent twin characteristic parameters was performed. The microstructure evolution of twins was investigated via a statistical viewpoint and in situ EBSD observation. 7
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Fig. 11. SF and ranking distribution of new nucleation twins in 1#-6-1-T-2 sample.
Acknowledgements
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The authors are grateful for the support from the National Natural Science Foundation of China (No. 51471038) and the National special support program for high-level personnel recruitment. References [1] J. Zhang, Y. Dou, G. Liu, Z. Guo, First-principles study of stacking fault energies in Mg-based binary alloys, Comput. Mater. Sci. 79 (2013) 564–569, https://doi.org/ 10.1016/j.commatsci.2013.07.012. [2] S.R. Agnew, J.F. Nie, Preface to the viewpoint set on: the current state of magnesium alloy science and technology, Scr. Mater. 63 (7) (2010) 671–673, https://doi. org/10.1016/j.scriptamat.2010.06.029. [3] L. Song, B. Wu, L. Zhang, X. Du, Y. Wang, C. Esling, Twinning characterization of fiber-textured AZ31B magnesium alloy during tensile deformation, Mater. Sci. Eng. A 710 (2018) 57–65, https://doi.org/10.1016/j.msea.2017.10.055. [4] K.K. Alaneme, E.A. Okotete, Enhancing plastic deformability of Mg and its alloys—a review of traditional and nascent developments, Journal of Magnesium & Alloys 5 (4) (2017), https://doi.org/10.1016/j.jma.2017.11.001. [5] S. You, Y. Huang, K.U. Kainer, N. Hort, Recent research and developments on wrought magnesium alloys, Journal of Magnesium & Alloys 5 (3) (2017), https:// doi.org/10.1016/j.jma.2017.09.001. [6] A. Levinson, R.K. Mishra, R.D. Doherty, S.R. Kalidindi, Influence of deformation twinning on static annealing of AZ31 Mg alloy, Acta Mater. 61 (16) (2013) 5966–5978, https://doi.org/10.1016/j.actamat.2013.06.037. [7] D. Sarker, D.L. Chen, Dependence of compressive deformation on pre-strain and loading direction in an extruded magnesium alloy: texture, twinning and de-twinning, Mater. Sci. Eng. A 596 (596) (2014) 134–144, https://doi.org/10.1016/j. msea.2013.12.038. [8] X.Y. Lou, M. Li, R.K. Boger, S.R. Agnew, R.H. Wagoner, Hardening evolution of AZ31B Mg sheet, Int. J. Plast. 23 (1) (2007) 44–86, https://doi.org/10.1016/j. ijplas.2006.03.005. [9] L. Jiang, J.J. Jonas, R.K. Mishra, A.A. Luo, A.K. Sachdev, S. Godet, Twinning and texture development in two Mg alloys subjected to loading along three different strain paths, Acta Mater. 55 (11) (2007) 3899–3910, https://doi.org/10.1016/j. actamat.2007.03.006. [10] S.G. Hong, S.H. Park, S.L. Chong, Role of {10–12} twinning characteristics in the deformation behavior of a polycrystalline magnesium alloy, Acta Mater. 58 (18) (2010) 5873–5885, https://doi.org/10.1016/j.actamat.2010.07.002. [11] M. Knezevic, A. Levinson, R. Harris, R.K. Mishra, R.D. Doherty, S.R. Kalidindi, Deformation twinning in AZ31: influence on strain hardening and texture evolution, Acta Mater. 58 (19) (2010) 6230–6242, https://doi.org/10.1016/j.actamat. 2010.07.041. [12] Y. Xin, M. Wang, Z. Zeng, G. Huang, Q. Liu, Tailoring the texture of magnesium alloy by twinning deformation to improve the rolling capability, Scr. Mater. 64 (10) (2011) 986–989, https://doi.org/10.1016/j.scriptamat.2011.02.010. [13] J.J. Jonas, S. Mu, T. Al-Samman, G. Gottstein, L. Jiang, Ė. Martin, The role of strain accommodation during the variant selection of primary twins in magnesium, Acta Mater. 59 (5) (2011) 2046–2056, https://doi.org/10.1016/j.actamat.2010.12.005. [14] K. Lu, L. Lu, S. Suresh, Strengthening materials by engineering coherent internal boundaries at the nanoscale, Science 324 (2009) 349–352, https://doi.org/10. 1126/science.1159610. [15] B. Song, R.L. Xin, G. Chen, X.Y. Zhang, Q. Liu, Improving tensile and compressive properties of magnesium alloy plates by pre-cold rolling, Scr. Mater. 66 (2012)
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effects of pre-strain on twinning behaviors in an extruded Mg-Zr alloy a
Xin Wan , Jing Zhang a b
a,b,∗
a
, Xueyan Mo , Yu Luo
T
a
College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China National Engineering Research Center for Magnesium Alloys, Chongqing, 400044, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Pre-strain Twin aspect ratio Twin density Twinning behaviors
For Mg alloys, regulation of size and quantity of twins is important to improve their performance. With respect to twin regulation, pre-strain (e.g., direct compression and compression–tension) is recognized as a reliable method. In the present study, the further investigation about the influence of different pre-strain loading conditions on the evolution of the subsequent twinning behavior was carried out via statistical viewpoint and in-situ EBSD observation. The results show that the dominant deformation mechanism can be changed by different prestrains. In the direct compression pre-strained sample, the subsequent deformation is dominated by twin growth. However, subsequent deformation is dominated by twin nucleation in the compression–tension pre-strained sample with complex pre-strain histories. Additionally, the results reveal that the compression–tension pre-strain is beneficial to promote the nucleation of twins inside grains, other than on the grain boundaries, and thereby increases the density of twins. Particularly, the pre-strain suggests that the nucleation proportion of twins inside the grain exceeds that at the grain boundary, and this characteristic significantly differs from the general view. Correspondingly, experimental results and the underlying mechanism of the aforementioned special phenomenon are reported and discussed.
1. Introduction Magnesium (Mg) and its alloys are lightweight structural materials that attract significant research attention in electronic communications, aerospace, and automotive industries [1–3]. However, their widespread application encountered bottlenecks due to their low strength and ductility [4,5]. Mg alloys exhibit a limited number of independent slip systems at room temperature. Hence, deformation twinning represents an extremely important deformation mechanism in terms of mechanical properties and deformation behaviors [6–8]. Among various twinning modes in Mg alloys, {10–12} extension twin is expected to occur most easily, and this is attributed to its significantly lower critical resolved shear stress (CRSS) [6,9,10]. The main role of twinning in plastic deformation includes reorientation of the twinned regions, relaxing stress concentrations, and Hall–Petch effect via twinning-induced change in grain size [11–13]. Recently, the introduction of fine and dense twin lamellae is demonstrated as an effective means to tailor the ductility and strength of metallic materials [14–16]. Lu et al. [14] revealed that pure electro-deposited copper with high-density nanotwins exhibits extremely high strength with significant plastic strain. Furthermore, Song et al. [15] and Xin et al. [16] reported that the fine {10–12} twin can effectively toughens and strengthens textured Mg alloys.
∗
The amount and size of {10–12} twins can be tailored by pre-strain paths and levels. For example, Ghaderi et al. [17] and Barnett et al. [18] investigated the effect of compressive strain on twin characteristic parameters and revealed that the twin aspect ratio (twin thickness over twin length) and twin number density increased with increasing strain within the considered strain range. Generally, detwinning behavior of the twinned area is easily activated during subsequent strain-pathchanged reloading or reverse loading [19,20]. Specifically, strains mediated by twinning can be absolutely recovered via detwinning during subsequent reverse loading [21]. Pre-strain paths can include direct compression and compression–tension. Differences in the complex initial microstructure are observed in different pre-strain loading conditions. Our recent study revealed that when twinning and detwinning behavior are combined, twin aspect ratio and twin incidence are regulated via controlling compression–tension pre-strain [22]. In addition to different amount and size of twins obtained via different pre-strain paths and levels, the dislocation density in the microstructure also exhibits significant differences. Although considerable efforts focused on investigating twin nucleation and evolution characteristics while considering different pre-stain paths and levels, there is paucity of investigations on how the complex initial microstructure introduced by different pre-strain loading conditions further affects subsequent
Corresponding author. College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China. E-mail address: [email protected] (J. Zhang).
https://doi.org/10.1016/j.msea.2019.138335 Received 14 June 2019; Received in revised form 24 August 2019; Accepted 26 August 2019 Available online 27 August 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
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solution of 10 mL acetic acid, 5 g picric acid, 70 mL ethanol for 10–15 s at ambient temperature. For each sample, five OM pictures were taken at different viewing areas of the sample. The average twin aspect ratio and twin density (N, per mm2) on the OM pictures were measured using a TCI metallographic analysis system that exhibits a very rich processing and measurement functions [22], and more than 2000 twins were extracted in each sample. For EBSD examination, a scanning electron microscope (JOEL JSM 7800F) equipped with an HKL Channel 5 System was employed. In order to reveal the deformation behavior via re-compression, in situ EBSD maps were utilized using a step size of 0.8 μm, and the magnification was set as 400. All EBSD measurements were conducted from the center of the samples. The observed surface was ground mechanically, followed by electrochemical polishing in commercial AC2 solution for 35 s at 20 V. The EBSD data were analyzed using the Channel 5 software.
twinning behaviors. It is noted that a systematic study on the effect of pre-strain loading conditions on twin evolution significantly contributes to an effective tailoring of twin microstructure. In this paper, direct compression of pre-strain and compression–tension pre-strain tests along the extrusion direction (ED) was conducted on extruded Mg-Zr rods to introduce different initial microstructures. Subsequently, re-compression tests along the ED were employed on the pre-strained samples to observe the subsequent microstructure evolution. The effects of pre-strain loading conditions on the variation in twin characteristic parameters (i.e., average twin aspect ratio and twin density) during re-compression were quantitatively investigated from a statistical viewpoint to examine the relationship between pre-strain loading conditions and subsequent twinning behavior. Furthermore, in situ electron backscatter diffraction (EBSD) tests were performed to further clarify the underlying mechanism of the twinning behavior in different pre-strained samples during re-compression.
3. Results 2. Experiments and methods 3.1. Mechanical behavior 2.1. Sample preparation and mechanical tests Stress–strain curves of the pre-strained samples under re-compression along the ED to a cumulative strain of 2% are given in Fig. 2. From these results, it can be seen that all pre-strained samples exhibit similar shapes, but the yield stresses are significantly different. The yield stresses derived from those stress–strain curves are listed in Table 2. Sample 1#-1-T experienced direct compression pre-strain; it is observed that the yield stress is at a relatively low level. Specimen 1#-2-1-T is pre-deformed via compression 2% followed by reverse tension 1%, and this led to an appreciable increment in strength (Δσ = 6 MPa). Additionally, as shown in Table 2, a higher CYSs are produced in the 1#-41-T sample (Δσ = 15 MPa) and 1#-6-1-T sample (Δσ = 20 MPa), compared to the 1#-2-1-T sample. The increment of yield stress is potentially attributed to the fact that the dislocation density in matrix which could inhibit the twin boundaries migration increases with the initial compression strain. Increment in the dislocation density increases the yield stress.
Hot-extruded Mg-0.037Zr (wt.%) alloy bar with a diameter of 16 mm was used in the present study. After annealing at 350 °C for 2 h, equiaxial twin-free grain structure with an average grain size of 29 μm was obtained in the material, as shown in Fig. 1(a). The initial texture of the material was measured using Rigaku D/Max–2500 PC X-ray diffraction machine. The results showed that the initial texture of the hot-extruded Mg bar exhibited a strong basal texture with majority of basal poles normal to the ED, as seen in Fig. 1(b). In the pre-strain tests, the specimens were firstly subjected to compression test, in which the specimens were initially compressed along ED to different plastic strain levels of 1%, 2%, 4% and 6%, and then reversely stretched or not, to a same accumulated compression strains of 1% immediately after unloading. Following this, a relatively low temperature annealing, 200 °C for 0.5 h, was conducted on the samples, which is unlikely to remove the majority of the dislocations. The purpose of this heat treatment is to reduce the internal stresses caused by complex deformation histories. In the re-compression tests, the pre-strain samples were re-compressed by 1% along the ED to a cumulative strain of 2%. The designations of the pre-strain and re-compression test samples, along with their deformation histories, are displayed in Table 1. All deformation tests were performed at a strain rate of 10−3 s−1 using an INSTRON dynamic fatigue testing machine at room temperature.
3.2. Microstructure evolution during re-compression The optical microstructures of pre-strained and re-compressed samples are shown in Fig. 3. As shown in Fig. 3(a1-d1), many lenticular residual twins (denoted in red) are observed after the pre-strain. Interestingly, the twin characteristics at the same 1% cumulative strain differ because pre-strained samples experience different deformation histories. The detailed statistics of twin characteristic parameters as calculated by TCI metallographic analysis system for different loading conditions are shown in Fig. 4. It is observed that the prior twins obtained by the compression–tension pre-strain are finer and exhibit a number density that exceeds those of the direct compression pre-strain. The optical microstructure of re-compressed samples are shown in
2.2. Microstructure and texture measurements Microstructure evolution on the cross section (RD-ND plane) was characterized by an optical microscope (Axiovert 40 MAT Zeiss) and an EBSD technique. The samples for optical microstructure (OM) examination were mechanically ground, and then chemically etched in a
Fig. 1. (a) Optical micrographs and (b) corresponding texture of the annealed extrusion alloy bar. TD-transverse direction; ND-normal direction. 2
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Table 1 Designations of the pre-strain and re-compression test samples. Sample
1#-1-T 1#-2-1-T 1#-4-1-T 1#-6-1-T 1#-1-T-2 1#-2-1-T-2 1#-4-1-T-2 1#-6-1-T-2
Pre-strain Compression strain (%)
Reverse tensile strain (%)
1 2 4 6 1 2 4 6
–a 1 3 5 –a 1 3 5
Intermediate Annealing
Re-compression (%)
Accumulated compression strain (%)
T T T T T T T T
–b –b –b –b 1 1 1 1
1 1 1 1 2 2 2 2
T indicates that the sample was subjected to intermediate annealing. a Indicates that the sample was not subjected to reverse tension deformation. b Indicates that the sample was not subjected to re-compression deformation.
the twins in 1#-1-T sample is evident during the subsequent loading as shown in Fig. 3(a1-a2). However, with respect to 1#-6-1-T sample, the growth of twins is slight although the twin density varies significantly as shown in Fig. 3(d1-d2). Additionally, as shown in Figs. 3 and 4, 1#6-1-T sample is more prone to obtain high-density fine twins than other samples after re-compression. Subsequently, in situ EBSD analysis of the microstructure is performed, and the crystallographic orientation of the samples is determined. Figs. 5 and 6 show the IPF maps before and after re-compression of 1#-1-T sample and 1#-6-1-T sample, respectively. As shown in the figures, many parallel twin lamellas appear in the pre-strained samples. The twin lamellas are identified as {10–12} extension twins. Contraction twins or double twins are barely observed. After re-compression, significant differences in the nucleation sites of twins were observed between 1#-1-T-2 sample and 1#-6-1-T-2 sample. In 1#-1-T-2 sample, most twins nucleate at the grain boundaries. Twin chains are also observed as denoted by black regions in Fig. 5(b). However, in 1#6-1-T-2 sample, there are many areas of fine twin nucleation inside the grain, as shown in black regions in Fig. 6(b).
Fig. 2. Re-compression stress–strain curves along the ED for pre-strained samples.
4. Discussion Table 2 Compression yield stress (CYS) for pre-strained samples.
4.1. Effects of pre-strain loading conditions on twinning behavior
Sample
1#-1-T
1#-2-1-T
1#-4-1-T
1#-6-1-T
CYS (MPa)
67
73
82
87
The variation in twin characteristic parameters of pre-strained samples before and after re-compression was analyzed to investigate the effect of pre-strain loading conditions on subsequent twin nucleation and growth. The results are shown in Fig. 7. As shown in the figure, increase in the average twin aspect ratio in 1#-1-T sample is approximately 0.0154, and this exceeds any other samples. This indicates that the twin growth in 1#-1-T sample is more evident than others.
Fig. 3(a2-d2). The results reveal that twin growth and new nucleated twins are observed for all samples although the extent of growth and number of new twins differ from each other. For example, the growth of
Fig. 3. (a1-d1) Optical microstructure of pre-strained samples under different loading conditions, (a2-d2) optical microstructure of the re-compressed samples. 3
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Fig. 4. Distribution of twin characteristic parameters. (a) Twin density distribution, (b) average twin aspect ratio distribution.
increase of the initial compression strain, subsequent deformation induced by the twin growth is gradually weakened, and new twin nucleation is gradually promoted. Eventually, in 1#-6-1-T sample, new twin nucleation is more likely to occur rather than twin growth, thereby indicating that subsequent deformation is dominated by new twin nucleation.
Additionally, with respect to compression–tension pre-strain samples, the increase in average twin aspect ratio gradually decreases when the initial compression strain increases. Specifically, increase in the average twin aspect ratio in 1#-6-1-T sample is extremely low and nearly half of that in 1#-1-T sample, thereby indicating an inconspicuous twin growth in 1#-6-1-T sample. In contrast to the variation in the average twin aspect ratio, the twin density increment in 1#-6-1-T sample is significant (800 N/mm2). Conversely, the twin density increment in 1#1-T sample is rare (220 N/mm2). This implies that the twin nucleation in 1#-6-1-T sample is more universal than that in 1#-1-T sample during subsequent loading. Thus, it is concluded that the variation in the twin parameters are closely related to pre-strain histories. When the pre-strain history is simple (1#-1-T), increase in the average twin aspect ratio is at a relatively high level, and increase in the twin density is at a relatively low level under re-compression. Thus, the subsequent deformation is dominant by twin growth. However, when compression–tension prestrain is employed (samples 1#-2-1-T, 1#-4-1-T, 1#-6-1-T), with the
4.2. Effects of pre-strain loading conditions on subsequent nucleation of twins 4.2.1. Nucleation site analysis of new nucleation twins As shown in in situ EBSD maps, different nucleation characteristic of twins are found in samples 1#-1-T-2 and 1#-6-1-T-2. In order to further illustrate the effect of pre-strain histories on subsequent nucleation, the nucleation sites of the new twins in the two strain paths (approximately 300 grains) are calculated based on the in situ EBSD data, and the statistical results are shown in Fig. 8. It is observed that the proportion of grain boundary nucleation in 1#-1-T sample during the re-
Fig. 5. In situ EBSD maps (a, b) and boundaries misorientation maps (c, d) of the samples: (a) and (c) 1#-1-T; (b) and (d) 1#-1-T-2. New twins are denoted by white arrows. 4
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Fig. 6. In situ EBSD maps (a, b) and boundaries misorientation maps (c, d) of the samples: (a) and (c) 1#-6-1-T; (b) and (d) 1#-6-1-T-2. The new twins are denoted by the white arrows.
different from the general view. Evidently, the more complex the path the sample experiences, the more dislocation it contains, i.e., the dislocation density in the sample increases from 1#-1-T sample to 1#-6-1-T sample. Sample 1#-6-1-T is pre-deformed via compression and tension. There are many dislocations inside grains, and they are expected to be effective sites for subsequent twin nucleation, and the nucleation sites even exceed that of the grain boundaries. Additionally, the aforementioned defects can hinder twin growth via interaction between dislocations and twin boundaries and improve yield stress [27,28]. Thus, dense and fine twins are obtained in
compression significantly exceeds grain internal nucleation, and this is consistent with the results of other studies [23–25]. The twins tend to nucleate at grain boundaries, and this is potentially due to two reasons. First, the grain boundary is prone to stress concentration during the deformation, and this creates high stress conditions to promote the nucleation of twins. Second, the grain boundary itself can generate a twin core via atomic rearrangement under certain conditions [26]. However, after 1#-6-1-T pre-strained sample is re-compressed, the proportion of grain internal nucleation for twins is nearly twice that of grain boundary nucleation. The special phenomenon is significantly
Fig. 7. Variation in twin characteristic parameters during re-compression. The orange arrows represent increases in the average twin aspect ratio, and the blue arrows represent the increase in twin density. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 5
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data are calculated, as shown in Fig. 9. In 1#-1-T sample, when a low number of twins are involved in the initial grains, the ratio of grain boundary nucleation is more than twice that of grain internal nucleation. Twin growth is easy in the sample during re-compression, and this leads to a twinning stress field capable of promoting twin nucleation at the grain boundaries via an interaction between existing twins and the grain boundaries as shown in Fig. 5. However, when the amount of prior twin increases, the proportion of grain boundary nucleation gradually decreases, and the proportion of internal nucleation gradually increases. When the number of prior twin lamellae exceeds 3, the ratio of grain internal nucleation to grain boundary nucleation does not exhibit a significant difference. The results indicate that in 1#-1-T sample when the initial grain contains a high number of twins, twin–twin interaction is prevalent and subsequent promotion of grain internal nucleation is achieved via the interaction stress field [31,32]. With respect to 1#-6-1-T sample, the sample is pre-deformed by compression corresponding to 6% and then reverse tension corresponding to 5%. When the initial grains do not contain a twin, the subsequent twin is more favorable for nucleation at the grain boundaries than matrix, and this is identical to that of 1#-1-T sample as shown in Fig. 9 (b). However, when 1 to 4 twins are involved in the grain, the proportion of the grain internal nucleation exceeds that of the grain boundary nucleation. Hence, it is expected that the prior twinning stress field significantly affects subsequent twin nucleation in the grain in 1#-6-1-T sample. When prior twins exist in the grains, subsequent twins are more likely to nucleate inside the grain under the action of the prior twinning stress field. However, grain boundaries become favorable nucleation sites for subsequent twins when the grain does not contain prior twins. In order to further illustrate the effect of prior twins and dislocation density on subsequent twinning behavior in different pre-strain paths, the evolution of two twins with similar Schmid factor (SF) in 1#-1-T sample and 1#-6-1-T sample is explored as shown in Fig. 10. The relationship between the prior twins and subsequent twin nucleation in 1#-1-T sample is shown in Fig. 10(a and b). The initial grain Ma contains a prior twin Ta0, and twin growth is evident during the subsequent loading as shown in Fig. 10(b). When Ta0 interacts with the grain boundary, the stress concentration caused by the interaction facilitates subsequent twin Tb nucleation at the grain boundary in Ma due to the high difference in the orientation of Ma and Mb grains. In 1#-6-1T sample, the relationship between the prior twins and subsequent twin nucleation is shown in Fig. 10(c and d). It is observed that grain Mc possesses a prior twin Tb0. During the re-compression, the prior twin Tb0 grows to a slight extent. Conversely, several twin nucleation
Fig. 8. Nucleation sites of new twins in different pre-strain samples under recompression.
sample 1#-6-1-T-2. However, sample 1#-1-T only experiences direct compression by 1% with a few defects in the microstructure, and this is disadvantageous to subsequent nucleation inside the grains relative to grain boundaries. Eventually, the proportion of grain boundary nucleation exceeds that of grain internal nucleation. Thus, it is concluded that differences in the dislocation density in pre-strain samples is a potentially important factor that affects subsequent twin nucleation. 4.2.2. Relationship between prior twins and subsequent nucleation of twins Stress fields close to twin tips and related local areas of an hcp polycrystal were explored by Abdolvand [29]. The results indicated that the stress field around the twin tips varied with the local neighborhood. Gu et al. [30] revealed that twins exhibited high residual stresses that significantly deviated from those in the matrix grains after compression–tension pre-strain. Furthermore, Shi et al. [31] revealed that the stress field around the twins in the pre-deformation grains is capable of affecting subsequent nucleation of twins. Therefore, it is inferred that in addition to the dislocation density, the presence of prior twins can also affect subsequent twin evolution. In order to reveal the effect of prior twins on subsequent twin formation under different prestrain paths, the prior twin numbers of the aforementioned two samples in the grains where the new twin nucleation based on the in situ EBSD
Fig. 9. Relationship between prior twin number and subsequent nucleation sites under the re-compression process (a) in 1#-1-T sample and (b) in 1#-6-1-T sample. 6
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Fig. 10. Effect of prior twins on subsequent twin nucleation under different pre-strain paths (a,b) is for 1#-1-T sample and (c, d) for 1#-6-1-T sample.
Several conclusions are reached as follows:
(Tb–Te) are observed around Tb0. Simultaneously, twin nucleation (Tf, Tg) is also observed at the grain boundary. Thus, it is concluded that the relatively complex strain path leads to many defects inside the grain, and the subsequent nucleation of twins is promoted around the existing twins along under the effect of the high twinning residual stresses. Hence, with respect to 1#-6-1-T sample, subsequent twins are more likely to nucleate inside the grains as opposed to boundaries only when prior twins are present in the grain.
(1) When 1#-1-T sample is re-compressed, the average twin aspect ratio increment in 1#-1-T sample is approximately 0.0154, and the twin density increment in 1#-1-T sample is approximately 220 N/ mm2. However, when 1#-6-1-T sample is re-compressed, the average twin aspect ratio increment in the sample is approximately half that of 1#-1-T sample, and the twin density increment in the sample is nearly four times that of 1#-1-T sample. (2) The introduction of different number of defects and twins via the change in the pre-strain path can affect the subsequent deformation mode activation. When the initial grain contains a low number of defects and twins, the subsequent deformation is dominated by twin growth. However, the subsequent deformation is dominant by twin nucleation when a high number of defects and twins are introduced into the initial grain. (3) In terms of individual twinning stress field, in 1#-1-T sample, the twinning stress field predominantly promotes subsequent twin nucleation at the grain boundaries via interaction between twins and grain boundaries. In 1#-6-1-T sample, a significant number of dislocations exist in the sample, and the high twinning stress field induced by complex pre-strain histories can improve the ability of subsequent twins to nucleate inside the grain although it does not affect the twin variant selection. Thus, in 1#-6-1-T-2 sample, the ratio of grain internal nucleation is twice that of grain boundary nucleation. (4) Following the re-loading after pre-strain, the twin density in the sample increases with increasing the initial compressive strain, while the twin size decreases with increasing the initial compressive strain. Eventually, high-density fine twins are obtained in 1#6-1-T2 sample.
4.2.3. SF analysis of new nucleation twins in 1#6-1-T-2 As described above, 1#-6-1-T-2 sample exhibits high density defects, and thus the twinning stress field generated from prior twins significantly affect subsequent twin nucleation. The SF characteristics of the new nucleation twins in the grains containing prior twins are then analyzed in 1#-6-1-T-2 sample to investigate the effect of twinning stress field and high density defects on subsequent twin variants selection, and the results are shown in Fig. 11. It is observed that the SF value of the new nucleation twins in 1#-6-1-T-2 sample is relatively high, and 85% of the SF values are distributed in the range of 0.3–0.5. Simultaneously, nearly 75% of the new nucleation twins correspond to Rank1 and Rank2. Hence, it is concluded that the newly nucleated twins also conform to the SF law. Specifically, in 1#-6-1-T sample, the prior twinning stress field and defects significantly affect the nucleation of subsequent twins while they do not significantly affect the selection of subsequent twin variants. Therefore, in a manner similar to 1#-6-1-T sample, most grains in 1#-6-1-T-2 sample also contain just a single variant as shown in Fig. 6. 5. Conclusions In the present study, a comparative study about the effect of different pre-strains on the evolution of the subsequent twin characteristic parameters was performed. The microstructure evolution of twins was investigated via a statistical viewpoint and in situ EBSD observation. 7
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Fig. 11. SF and ranking distribution of new nucleation twins in 1#-6-1-T-2 sample.
Acknowledgements
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